Sometimes it works if the friend starts the convo - then you can message him back.
Sometimes one of the parties needs to disconnect and reconnect again, and then jump to another system.
Sometimes it helps to disable UPnP in Appconfig, run the game once, re-enable it and try again.
Sometimes nothing helps.
15+ ppl in the group - and most people experience the issue daily.
Not directly a firewall issue, since many people can resolve the issue restarting the game / tweaking appconfig and doing the daily ritual of checking friends-list, groups etc. Some doesn't run a firewall at their PC - just a router with UPnP enabled.
Voodoo?